1. FIELD OF THE INVENTION
The present invention relates to a thin film silicon semiconductor device which is important as a constituent element of a three-dimensional integrated circuit and a switching element for a flat panel display or the like and a process for producing the same, and particularly to a thin film silicon semiconductor device having excellent characteristics and a process for producing the same.
2. DESCRIPTION OF THE PRIOR ART
Thin film silicon semiconductor devices have recently attracted attention particularly as a constituent element of a three-dimensional integrated circuit and a switching element for a flat panel display, and hence are under extensive studies. Such semiconductor devices are reported in detail in a paper of D. S. Malhi et al. (IEEE Trans, Electron Devices ED-32 (1985) pp. 258-281). Thin film silicon semiconductor devices as used in the above-mentioned applications, when in the form of a field-effect transistor, comprise a thin silicon film of 0.05 to 2.0 .mu.m in thickness deposited as the basic constituent on an insulating substrate. Among them, those having a coplanar structure or a staggered structure are most widely used. FIG. 1 is a cross-sectional view of a thin film silicon semiconductor device having a coplanar structure. This semiconductor device has a structure comprising an insulating film 2 formed on the surface of an insulating substrate 1 and a series of a thin silicon film 3, a gate insulating film 4 and a gate electrode 5 laminated thereon in this order, plus, source electrode/drain electrode 6 for output power and metallic wiring 7. FIG. 2 is a cross-sectional view of a thin film silicon semiconductor device having a staggered structure. This semiconductor device comprises a gate electrode 5, a gate insulating film 4, and a thin silicon film 3 laminated in this order on an insulating film 2. Source electrode/drain electrode 6 are formed on the gate insulating film 4. When a positive or negative voltage is applied to the gate electrode 5, carriers are induced in the inside of the thin silicon film 3, particularly near the interface of the thin silicon film 3 with the gate insulating film 4, and flow between the source electrode and the drain electrode to develop an output voltage between the metallic wiring 7, whereby this thin film silicon semiconductor device is actuated. As is apparent from the above description, the characteristics of this semiconductor device are greatly affected by the properties of the thin silicon film.
A thermal chemical vapor deposition method (thermal CVD method) basically comprising thermal decomposition of a gas comprising silane (SiH.sub.4), disilane (Si.sub.2 H.sub.6), or the like as the main raw material, and a plasma chemical vapor deposition method (plasma CVD method) utilizing plasma and capable of easily effecting a treatment at a lower substrate temperature than the former method have heretofore been employed for the formation of a thin silicon film in a thin film silicon semiconductor device. The plasma CVD method, which involves a lower temperature at the time of film deposition than the thermal CVD method, is capable of depositing a thin silicon film even at 400.degree. C. or below and hence allows the use of an inexpensive substrate such as glass, and is employed for the production of a switching element for a flat panel display using a liquid crystal. Since a thin silicon film formed by this method is amorphous, however, the carrier mobility is as low as 1 cm.sup.2 /V.multidot.s, resulting in a difficulty in obtaining a high-performance thin film silicon semiconductor device.
On the other hand, since the thermal CVD method involves a higher substrate temperature than the plasma CVD method, it produces a silicon film not in an amorphous state but in a polycrystalline state. Since the substrate temperature in the thermal CVD method is 600.degree. to 700.degree. C. which is by far lower than the melting point of silicon, of 1,412.degree. C. however, the resulting silicon film is at best in a polycrystalline state of fine crystals. Therefore, the carrier mobility in a thin film silicon semiconductor device produced by the thermal CVD method is at most about 10 cm.sup.2 /V.multidot.s, though the performance thereof is superior to that of a thin film silicon semiconductor device comprising a thin silicon film in an amorphous state formed by the plasma CVD method. Thus, the thin polycrystalline silicon film formed by the thermal CVD method also has a greatly limited scope of applications due to the substantially higher substrate temperature and the comparatively low carrier mobility.
The reason for the poor characteristics of semiconductor devices comprising a thin silicon film formed by the above-mentioned thermal or plasma CVD method resides in the amorphous or microcrystalline state, made of extremely fine crystal grains, of the thin silicon film formed by the above-mentioned method.
A common method for solving the above-mentioned problems and giving excellent characteristics to the resulting semiconductor device comprises a heat treatment after the deposition of a thin silicon film to grow crystal grains therein. Specifically, a thin silicon film is deposited by a method utilizing a chemical reaction, such as the CVD method, and is subsequently crystallized by a laser annealing method or an annealing method comprising heating the film in a furnace at a high temperature for a long time to effect the growth of into large crystal grains having a size of 1 .mu.m or larger. Even where a large crystal grain size is provided by the above-mentioned method, the carrier mobility is at most 100 cm.sup.2 /V.multidot.s, which cannot be said to be large enough to enable the resulting semiconductor device to be applied to a wide variety of fields.
The above-mentioned conventional thin film silicon semiconductor devices further involve other problems in addition to the problem that high performance characteristics cannot be obtained. For a thin silicon film formed by the plasma CVD method, a trouble of the film exfoliating from the substrate frequently occurs. This results from the low substrate temperature at the time of film deposition which allows a large amount of unreacted silane gas, which is most widely used, hydrogen gas, and the like to remain in the film, with the result that these gases are released from the film during the subsequent processing of the thin film silicon semiconductor device. By contrast, the thermal CVD method does not cause the trouble of the resulting thin silicon film exfoliating from a substrate in the process for forming a thin polycrystalline silicon film thereby and in the heat-treating of the resulting thin film to provide a large crystal grain size. In this case, however, a substrate incapable of resisting high temperatures (e.g., glass) cannot be used for forming a thin film silicon semiconductor device thereon and a semiconductor device already formed the inside of a substrate is deteriorated in characteristics or broken in the case of a three-dimensional integrated circuit because the substrate is exposed to high temperatures in the above-mentioned processes.
Further, a thin film silicon semiconductor device having comparatively excellent characteristics and comprising a thin silicon film having large crystal grains involves the problem of a large lot-to-lot variation of characteristics. This arises from the size of the semiconductor device compared to the crystal grain size. FIG. 3 is a plan view of a channel region having a source electrode 8 and a drain electrode 9. When crystal grains 10 are large as shown in the figure, the number and positions of crystal grain boundaries present in a channel region which boundaries hinder transportation of carriers differ from semiconductor device to semiconductor device, resulting in a large lot-to-lot variation of characteristics. This is discussed in detail with specific examples in, for example, a paper of K. K. Ng et al. (IEEE Electron Device Letters, EDL-2, 1981, pp. 316-318).
When a thin polycrystalline silicon film is processed by etching according to a wet or dry process, crystal grain boundaries are substantially corroded as compared with the insides of crystal grains. In the case of a conventional polycrystalline silicon semiconductor device comprising a thin polycrystalline silicon film having large crystal grains, therefore, a difficulty is encountered in sharply processing the sides of the pattern of the thin polycrystalline silicon film as shown in FIG. 3. Thus, it has been difficult to produce fine thin film silicon semiconductor devices in high yield.
As described above, the conventional techniques involve the problems that it is difficult to obtain a high-performance thin film silicon semiconductor device by a process involving a low temperature, and that the production yield is low.